Chapter 23.5, Problem 13PTS

(a)

Interpretation Introduction

Interpretation: Using a Gabriel synthesis, a give set of primary amine compounds have to be synthesized along with their corresponding starting materials.

Concept Introduction: The general formula for primary amine is –NH₂.  There are several methods available to prepare primary amines.  Among them, Gabriel synthesis plays a very important role for preparing it.  In this method, secondary and tertiary amines are not formed as side products.  It involves in three steps.

Step-1:  Formation of potassium phthalimide (deprotonation)

Potassium phthalimide in alkaline KOH acts as the reagent which has negatively charged phthalimide.  It is formed by the reaction between phthalimide and potassium hydroxide.

Step-2: Formation of R−N bond by S_N2 nucleophilic substitution

The negative charged nitrogen atom in phthalimide can easily attract the positive side of R−X.  In primary alkyl halides (R−X), R and X get positive and negative charges, respectively when they ionize.  As a result, a bond between nitrogen of phthalimide and carbon of R is formed.  This is S_N2 nucleophilic substitution reaction.  Halogen atom is going away as halide anion.

Step-3: Formation of primary amine by hydrolysis

The resultant product further goes for hydrolysis using hydrazine as the reagent.  This reaction also follows nucleophilic substitution reaction.  Finally, primary amine is formed with a side product of hydrazine derivative.

The starting material of Gabriel synthesis is alkyl halide.  It can be prepared in a number of ways such as bromination-elimination-addition reactions, reduction-displacement of function group reactions, direct bromination reactions, etc.  Some of the reagents such as HBr, bromine in photolytic condition, phosphorous tribromide are used to achieve the halide group preparation.

(b)

Interpretation Introduction

Interpretation: Using a Gabriel synthesis, a give set of primary amine compounds have to be synthesized along with their corresponding starting materials.

Concept Introduction: The general formula for primary amine is –NH₂.  There are several methods available to prepare primary amines.  Among them, Gabriel synthesis plays a very important role for preparing it.  In this method, secondary and tertiary amines are not formed as side products.  It involves in three steps.

Step-1:  Formation of potassium phthalimide (deprotonation)

Potassium phthalimide in alkaline KOH acts as the reagent which has negatively charged phthalimide.  It is formed by the reaction between phthalimide and potassium hydroxide.

Step-2: Formation of R−N bond by S_N2 nucleophilic substitution

The negative charged nitrogen atom in phthalimide can easily attract the positive side of R−X.  In primary alkyl halides (R−X), R and X get positive and negative charges, respectively when they ionize.  As a result, a bond between nitrogen of phthalimide and carbon of R is formed.  This is S_N2 nucleophilic substitution reaction.  Halogen atom is going away as halide anion.

Step-3: Formation of primary amine by hydrolysis

The resultant product further goes for hydrolysis using hydrazine as the reagent.  This reaction also follows nucleophilic substitution reaction.  Finally, primary amine is formed with a side product of hydrazine derivative.

The starting material of Gabriel synthesis is alkyl halide.  It can be prepared in a number of ways such as bromination-elimination-addition reactions, reduction-displacement of function group reactions, direct bromination reactions, etc.  Some of the reagents such as HBr, bromine in photolytic condition, phosphorous tribromide are used to achieve the halide group preparation.